Combine harvesters are widely used in the harvesting of various types of grain. Typically, a combine harvester comprises a cutter bar, which cuts the crop from the field. The harvested crop is threshed inside the combine harvester. The stalks of the harvested crop are generally removed from the combine harvester via a straw walker and a mixture of grain and smaller particles of material other than grain remains in the combine harvester.
This mixture is transported to a cleaning shoe, in which the threshed grain is separated from the material other than grain. The cleaning shoe generally comprises one or more sieves, which perform a reciprocating movement during use. The sieved grain is then collected and transported to the grain tank of the combine harvester, which is generally emptied periodically.
The sieve or sieves of the cleaning shoe are generally arranged at an angle relative to the horizontal, with the front end of the sieve (that is, the end closest to the cutter bar) lower than the rear end of the sieve. The reciprocating movement of the sieve makes that the grain is thrown upwards and backwards by the sieve. A fan blows air over and through the sieve, to catch the lighter particles of the material other than grain and keep them airborne until they are blown out of the combine harvester.
The effectiveness of the sieving process is dependent on many variables of the harvesting process and the harvesting conditions, like the particle size of the mixture, grain size, moisture content and the angle of the terrain over which the combine harvester moves. For example, when the combine harvester moves downhill, the angle of the sieve relative to the horizontal will become steeper, with the result that the mixture of grain and material other than grain travels over the sieve slower and more material accumulates on the sieve. On the other hand, when the combine harvester moves uphill, the angle of the sieve relative to the horizontal will become flatter, with the result that mixture of grain and material other than grain travels over the sieve faster and more grain leaves the combine harvester with the material other than grain.
EP 1712122 proposes to adjust the frequency and the stroke of the sieve in order to match the movement of the sieve with the actual parameters of the harvesting process by means of applying linear actuators. These linear actuators are for example cylinders or magnetic actuators.
The known system of EP 1712122 however has serious durability problems in the harsh working conditions inside a combine harvester.